Planetary gear train of automatic transmission for vehicle

ABSTRACT

A planetary gear train of an automatic transmission for a vehicle may include an input shaft, a first planetary gear set including a first sun gear, a first planetary carrier, and a first ring gear, a second planetary gear set including a second sun gear, a second planetary carrier, and a second ring gear, a third planetary gear set including a third sun gear, a third planetary carrier, and a third ring gear, a fourth planetary gear set including a fourth sun gear, a fourth planetary carrier, and a fourth ring gear, a first rotational shaft, a second rotational shaft, a third rotational shaft, a fourth rotational shaft, a fifth rotational shaft, a sixth rotational shaft, a seventh rotational shaft, and an eighth rotational shaft.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No.10-2014-0177361 filed Dec. 10, 2014, the entire contents of which isincorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an automatic transmission for avehicle. More particularly, the present invention relates to a planetarygear train of an automatic transmission for a vehicle that improvespower delivery performance and fuel economy by achieving nine forwardspeeds using minimized constituent elements.

2. Description of Related Art

In recent years, a rise in oil price causes dashing into unlimitedcompetition for enhancing fuel efficiency.

As a result, research into reduction of weight and the enhancement offuel efficiency through down-sizing are conducted in the case of anengine and research for simultaneously securing operability and fuelefficiency competitiveness through multi-speeds are conducted in thecase of an automatic transmission.

However, in the automatic transmission, as the number of speedsincrease, the number of internal components increase, and as a result,mountability, cost, weight, transmission efficiency, and the like maystill deteriorate.

Accordingly, development of a planetary gear train which may bring aboutmaximum efficiency with a small number of components may be important inorder to increase a fuel efficiency enhancement effect through themulti-stages.

In this aspect, in recent years, 8 and 9-speed automatic transmissionstend to be implemented and the research and development of a planetarygear train capable of implementing more transmission steps has also beenactively conducted.

The information disclosed in this Background of the Invention section isonly for enhancement of understanding of the general background of theinvention and should not be taken as an acknowledgement or any form ofsuggestion that this information forms the prior art already known to aperson skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing aplanetary gear train of an automatic transmission for a vehicle thatimplements nine forward speeds and one reverse speed with a minimumconfiguration and reduces drag torque by minimizing non-operatedfriction elements while three friction elements operate in respectivegear stages to improve power transmission performance and fuelefficiency.

According to various aspects of the present invention, a planetary geartrain of an automatic transmission for a vehicle may include an inputshaft into which power of an engine is input, a first planetary gear setincluding a first sun gear, a first planetary carrier, and a first ringgear, a second planetary gear set including a second sun gear, a secondplanetary carrier, and a second ring gear, a third planetary gear setincluding a third sun gear, a third planetary carrier, and a third ringgear, a fourth planetary gear set including a fourth sun gear, a fourthplanetary carrier, and a fourth ring gear, a first rotational shaftincluding the first sun gear, selectively connected to the input shaft,and selectively connected to a transmission housing, a second rotationalshaft including the first planet carrier and the second ring gear, andselectively connected to the transmission housing, a third rotationalshaft including the second planet carrier, a fourth rotational shaftincluding the second sun gear and directly connected to the input shaft,a fifth rotational shaft including the first ring gear and the fourthsun gear, and selectively connected to the third rotational shaft, asixth rotational shaft including the third ring gear and the fourthplanet carrier, and selectively connected to the input shaft, a seventhrotational shaft including the third planet carrier and the fourth ringgear or the fourth ring gear only, and directly connected to an outputshaft, and an eighth rotational shaft including the third sun gear anddirectly connected to the transmission housing.

Each of the first, second, third, and fourth planetary gear sets may bea single pinion planetary gear set.

The planetary gear train may further include a first clutch interposedbetween the input shaft and the first rotational shaft, a second clutchinterposed between the third planet carrier and the fourth ring gear, athird clutch interposed between the second rotational shaft and thefifth rotational shaft, a fourth clutch interposed between the inputshaft and the sixth rotational shaft, a first brake interposed betweenthe first rotational shaft and the transmission housing, and a secondbrake interposed between the second rotational shaft and thetransmission housing.

A first forward speed may be achieved by operation of the second brakeand the second and third clutches, a second forward speed may beachieved by operation of the first brake and the second and thirdclutches, a third forward speed may be achieved by operation of thefirst, second, and third clutches, a fourth forward speed may beachieved by operation of the second, third, and fourth clutches, a fifthforward speed may be achieved by operation of the first, third, andfourth clutches, a sixth forward speed may be achieved by operation ofthe first brake and the third and fourth clutches, a seventh forwardspeed may be achieved by operation of the second brake and the third andfourth clutches, an eighth forward speed may be achieved by operation ofthe first and second brakes and the fourth clutch, a ninth forward speedmay be achieved by operation of the second brake and the first andfourth clutches, and a reverse speed may be achieved by operation of thesecond brake and the first and second clutches.

According to various aspects of the present invention, a planetary geartrain of an automatic transmission for a vehicle may include an inputshaft into which power of an engine is input, a first planetary gear setincluding a first sun gear, a first planetary carrier, and a first ringgear, a second planetary gear set including a second sun gear, a secondplanetary carrier, and a second ring gear, a third planetary gear setincluding a third sun gear, a third planetary carrier, and a third ringgear, and a fourth planetary gear set including a fourth sun gear, afourth planetary carrier, and a fourth ring gear, in which the first sungear may be selectively connected to the input shaft and may beselectively connected to a transmission housing, the first planetcarrier and the second ring gear that are directly connected to eachother are selectively connected to the transmission housing, the secondsun gear may be directly connected to the input shaft, the first ringgear and the fourth sun gear that are directly connected to each otherare selectively connected to the second planet carrier, the third ringgear and the fourth planet carrier that are directly connected to eachother are selectively connected to the input shaft, the third planetcarrier may be selectively connected to the fourth ring gear and anoutput shaft that are directly connected to each other, and the thirdsun gear may be directly connected to the transmission housing.

Various embodiments of the present invention may achieve nine forwardspeeds by combining four planetary gear sets that are simple planetarygear sets with six frictional elements. Therefore, power deliveryperformance and fuel economy may be improved.

In addition, three friction elements operate at each speed, and as aresult, a friction drag loss is decreased by minimizing the number offriction elements which do not operate. Therefore, drag torque and powerloss may be reduced.

It is understood that the term “vehicle” or “vehicular” or other similarterms as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g., fuel derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example, bothgasoline-powered and electric-powered vehicles.

The methods and apparatuses of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated herein, andthe following Detailed Description, which together serve to explaincertain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary planetary gear trainaccording to the present invention.

FIG. 2 is an operation chart of frictional elements at each speed in theexemplary planetary gear train according to the present invention.

FIG. 3 is a lever diagram of an exemplary planetary gear train accordingto the present invention.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the present invention as disclosed herein,including, for example, specific dimensions, orientations, locations,and shapes will be determined in part by the particular intendedapplication and use environment.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying drawings and described below. While the invention(s) willbe described in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit theinvention(s) to those exemplary embodiments. On the contrary, theinvention(s) is/are intended to cover not only the exemplaryembodiments, but also various alternatives, modifications, equivalentsand other embodiments, which may be included within the spirit and scopeof the invention as defined by the appended claims.

FIG. 1 is a schematic diagram of a planetary gear train according tovarious embodiments of the present invention.

Referring to FIG. 1, a planetary gear train according to variousembodiments of the present invention includes first, second, third, andfourth planetary gear sets PG1, PG2, PG3, and PG4 disposed on the sameaxis, an input shaft IS, an output shaft OS, eight rotational shafts TM1to TM8 that directly connect any one or two rotation elements of thefirst, second, third, and fourth planetary gear sets PG1, PG2, PG3, andPG4, six frictional elements C1 to C4 and B1 to B2, and a transmissionhousing H.

As a result, power input from the input shaft IS is changed bycooperation of the first, second, third, and fourth planetary gear setsPG1, PG2, PG3, and PG4, and the changed power is output through theoutput shaft OS.

In addition, the simple planetary gear sets are disposed in a sequenceof the first, second, third, and fourth planetary gear sets PG1, PG2,PG3, and PG4 from an engine side.

The input shaft IS is an input member and power from a crankshaft of anengine is torque-converted through a torque converter to be input intothe input shaft IS.

The output shaft OS is an output member and transmits driving power to adriving wheel through a differential apparatus.

The first planetary gear set PG1 is a single pinion planetary gear setand includes a first sun gear S1, a first planet carrier PC1 rotatablysupporting a first pinion P1 that is externally meshed with the firstsun gear S1, and a first ring gear R1 that is internally meshed with thefirst pinion P1 as rotation elements thereof.

The second planetary gear set PG2 is a single pinion planetary gear setand includes a second sun gear S2, a second planet carrier PC2 rotatablysupporting a second pinion P2 that is externally meshed with the secondsun gear S2, and a second ring gear R2 that is internally meshed withthe second pinion P2 as rotation elements thereof.

The third planetary gear set PG3 is a single pinion planetary gear setand includes a third sun gear S3, a third planet carrier PC3 rotatablysupporting a third pinion P3 that is externally meshed with the thirdsun gear S3, and a third ring gear R3 that is internally meshed with thethird pinion P3 as rotation elements thereof.

The fourth planetary gear set PG4 is a single pinion planetary gear setand includes a fourth sun gear S4, a fourth planet carrier PC4 rotatablysupporting a fourth pinion P4 that is externally meshed with the fourthsun gear S4, and a fourth ring gear R4 that is internally meshed withthe fourth pinion P4 as rotation elements thereof.

In the first, second, third, and fourth planetary gear sets PG1, PG2,PG3, and PG4, one or more rotation elements are directly connected toeach other to operate with eight rotational shafts TM1 to TM8.

Configurations of the eight rotational shafts TM1 to TM8 will bedescribed below.

The first rotational shaft TM1 includes the first sun gear S1, isselectively connected to the input shaft IS, and is selectivelyconnected to the transmission housing H so as to be operated as aselective input element or a selective fixed element.

The second rotational shaft TM2 includes the first planet carrier PC1and the second ring gear R2, and is selectively connected to thetransmission housing H so as to be operated as a selective fixedelement.

The third rotational shaft TM3 includes the second planet carrier PC2.

The fourth rotational shaft TM4 includes the second sun gear S2 and isdirectly connected to the input shaft IS so as to be always operated asan input element.

The fifth rotational shaft TM5 includes the first ring gear R1 and thefourth sun gear S4, and is selectively connected to the third rotationalshaft TM3.

The sixth rotational shaft TM6 includes the third ring gear R3 and thefourth planet carrier PC4, and is selectively connected to the inputshaft IS so as to be operated as a selective input element.

The seventh rotational shaft TM7 includes the third planet carrier PC3and the fourth ring gear R4, or the fourth ring gear R4 only, and isdirectly connected to the output shaft OS so as to be operated as afinal output element.

A friction element is disposed between the third planet carrier PC3 andthe fourth ring gear R4 such that the seventh rotational shaft TM7includes the third planet carrier PC3 and the fourth ring gear R4 if thefriction element operates, but the seventh rotational shaft TM7 includesthe fourth ring gear R4 only if the friction element does not operate.

The eighth rotational shaft TM8 includes the third sun gear S3, and isdirectly connected to the transmission housing H so as to be alwaysoperated as a fixed element.

In addition, among the rotational shafts TM1 to TM8, four clutches C1,C2, C3, and C4 which are friction elements are disposed at the inputshaft IS or portions where the rotational shafts are selectivelyconnected to each other.

Further, among the rotational shafts TM1 to TM8, two brakes B1 and B2which are the friction elements are disposed at portions selectivelyconnected to the transmission housing H.

The first clutch C1 is interposed between the input shaft IS and thefirst rotational shaft TM1 so as to selectively connect the input shaftIS to the first rotational shaft TM1.

The second clutch C2 is interposed between the third planet carrier PC3and the fourth ring gear R4 so as to cause the third planet carrier PC3and the fourth ring gear R4 to form the seventh rotational shaft TM7 orto cause only the fourth ring gear R4 to form the seventh rotationalshaft TM7.

The third clutch C3 is interposed between the third rotational shaft TM3and the fifth rotational shaft TM5 so as to selectively connect thethird rotational shaft TM3 to the fifth rotational shaft TM5.

The fourth clutch C4 is interposed between the input shaft IS and thesixth rotational shaft TM6 so as to connect the input shaft IS to thesixth rotational shaft TM6.

The first brake B1 is interposed between the first rotational shaft TM1and the transmission housing H so as to operate the first rotationalshaft TM1 as the selective fixed element.

The second brake B2 is interposed between the second rotational shaftTM2 and the transmission housing H so as to operate the secondrotational shaft TM2 as the selective fixed element.

The frictional elements including the first, second, third, and fourthclutches C1, C2, C3, and C4 and the first and second brake B1 and B2 maybe multi-plate friction elements of wet type that are operated byhydraulic pressure.

FIG. 2 is an operation chart of frictional elements at each speed in theplanetary gear train according to various embodiments of the presentinvention.

As shown in FIG. 2, three frictional elements are operated at each speedin the planetary gear train according to various embodiments of thepresent invention.

A first forward speed D1 is achieved by operation of the second brake B2and the second and third clutches C2 and C3.

A second forward speed D2 is achieved by operation of the first brake B1and the second and third clutches C2 and C3.

A third forward speed D3 is achieved by operation of the first, second,and third clutches C1, C2, and C3.

A fourth forward speed D4 is achieved by operation of the second, third,and fourth clutches C2, C3, and C4.

A fifth forward speed D5 is achieved by operation of the first, third,and fourth clutches C1, C3, and C4.

A sixth forward speed D6 is achieved by operation of the first brake B1and the third and fourth clutches C3 and C4.

A seventh forward speed D7 is achieved by operation of the second brakeB2 and the third and fourth clutches C3 and C4.

An eighth forward speed D8 is achieved by operation of the first andsecond brakes B1 and B2 and the fourth clutch C4.

A ninth forward speed D9 is achieved by operation of the second brake B2and the first and fourth clutches C1 and C4.

A reverse speed REV is achieved by operation of the second brake B2 andthe first and second clutches C1 and C2.

FIG. 3 is a lever diagram of a planetary gear train according to variousembodiments of the present invention. Shifting processes of theplanetary gear train according to the various embodiments of the presentinvention are illustrated through a lever analysis method.

Referring to FIG. 3, eight vertical lines are set as the firstrotational shaft TM1, the second rotational shaft TM2, the thirdrotational shaft TM3, the fourth rotational shaft TM4, the fifthrotational shaft TM5, the sixth rotational shaft TM6, the seventhrotational shaft TM7, and the eighth rotational shaft TM8 from the leftto the right.

In addition, a lower horizontal line represents rotation speed of “0”,and an upper horizontal line represents rotation speed of “1”, that isthe same rotation speed as the input shaft IS.

Intervals between the vertical lines are set according to each gearratio (the teeth number of sun gear/the teeth number of ring gear) ofeach of the planetary gear sets PG1 to PG4.

Further, since setting of a rotational axis of the vertical line whichis set by inter-combinations of the planetary gear sets is, of course,known to those skilled in the art of the planetary gear train, adetailed description thereof will be omitted.

Meanwhile, the seventh rotational shaft TM7 includes the third planetcarrier PC3 and the fourth ring gear R4 at the first, second, third, andfourth forward speeds and the reverse speed where the second clutch C2operates, and includes only the fourth ring gear R4 at the fifth, sixth,seventh, eighth, and ninth forward speeds where the second clutch C2does not operate.

If the second clutch C2 does not operate, the third planet carrier PC3runs idle. So, the third planet carrier PC3 rotates but does not anyeffect on the shifting.

Referring to FIG. 2 and FIG. 3, shifting processes for each speed in theplanetary gear train according to various embodiments of the presentinvention will be described

[First Forward Speed]

Referring to FIG. 2, the second brake B2 and the second and thirdclutches C2 and C3 are operated at the first forward speed D1.

As shown in FIG. 3, in a state that a rotation speed of the input shaftIS is input to the fourth rotational shaft TM4, the second rotationalshaft TM2 is operated as the fixed element by operation of the secondbrake B2. Therefore, the rotation elements of the first and secondplanetary gear sets PG1 and PG2 form a first speed line T1 and changedrotation speed is output through the third rotational shaft TM3.

In addition, in a state that rotation speed of the third rotationalshaft TM3 is input to the fifth rotational shaft TM5 by operation of thethird clutch C3, the eighth rotational shaft TM8 is operated as thefixed element. Therefore, the rotation elements of the third and fourthplanetary gear sets PG3 and PG4 form a first shift line SP1, and D1 isoutput through the seventh rotational shaft TM7 that is the outputelement.

[Second Forward Speed]

The second brake B2 that was operated at the first forward speed D1 isreleased and the first brake B1 is operated at the second forward speedD2.

As shown in FIG. 3, in a state that the rotation speed of the inputshaft IS is input to the fourth rotational shaft TM4, the firstrotational shaft TM1 is operated as the fixed element by operation ofthe first brake B1. Therefore, the rotation elements of the first andsecond planetary gear sets PG1 and PG2 form a second speed line T2 andchanged rotation speed is output through the third rotational shaft TM3.

In addition, in a state that rotation speed of the third rotationalshaft TM3 is input to the fifth rotational shaft TM5 by operation of thethird clutch C3, the eighth rotational shaft TM8 is operated as thefixed element. Therefore, the rotation elements of the third and fourthplanetary gear sets PG3 and PG4 form a second shift line SP2, and D2 isoutput through the seventh rotational shaft TM7 that is the outputelement.

[Third Forward Speed]

The first brake B1 that was operated at the second forward speed D2 isreleased and the first clutch C1 is operated at the third forward speedD3.

As shown in FIG. 3, in a state that the rotation speed of the inputshaft IS is input to the fourth rotational shaft TM4, the rotation speedof the input shaft IS is input to the first rotational shaft TM1 byoperation of the first clutch C1. Therefore, the first and secondplanetary gear sets PG1 and PG2 become direct-coupling state and therotation elements of the first and second planetary gear sets PG1 andPG2 form a third speed line T3.

In addition, in a state that rotation speed of the third rotationalshaft TM3 is input to the fifth rotational shaft TM5 by operation of thethird clutch C3, the eighth rotational shaft TM8 is operated as thefixed element. Therefore, the rotation elements of the third and fourthplanetary gear sets PG3 and PG4 form a third shift line SP3, and D3 isoutput through the seventh rotational shaft TM7 that is the outputelement.

[Fourth Forward Speed]

The first clutch C1 that was operated at the third forward speed D3 isreleased and the fourth clutch C4 is operated at the fourth forwardspeed D4.

As shown in FIG. 3, the rotation speed of the input shaft IS is input tothe sixth rotational shaft TM6 by operation of the fourth clutch C4, theeighth rotational shaft TM8 is operated as the fixed element. Therefore,the rotation elements of the third and fourth planetary gear sets PG3and PG4 form a fourth shift line SP4, and D4 is output through theseventh rotational shaft TM7 that is the output element.

At this time, in a state that the rotation speed of the input shaft ISis input to the fourth rotational shaft TM4, the third rotational shaftTM3 is connected to the fifth rotational shaft TM5 by operation of thethird clutch C3. Therefore, the rotation elements of the first andsecond planetary gear sets PG1 and PG2 form a fourth speed line T4, buthave no effects on the shifting.

[Fifth Forward Speed]

The second clutch C2 that was operated at the fourth forward speed D4 isreleased and the first clutch C1 is operated at the fifth forward speedD5.

As shown in FIG. 3, the rotation speed of the input shaft IS is input tothe first rotational shaft TM1, the fourth rotational shaft TM4, and thesixth rotational shaft TM6 by operation of the first, third, and fourthclutches C1, C3, and C4. Therefore, the first, second, third, and fourthplanetary gear sets PG1, PG2, PG3, and PG4 become direct-couplingstates, the rotation elements of the first and second planetary gearsets PG1 and PG2 form the third speed line T3, and the rotation elementsof the third and fourth planetary gear sets PG3 and PG4 form a fifthshift line SP5. Therefore, D5 is output through the seventh rotationalshaft TM7 that is the output element.

At this time, the eighth rotational shaft TM8 is operated as the fixedelement, but the third planet carrier PC3 runs idle and has no effectson the shifting since the second clutch C2 is released.

[Sixth Forward Speed]

The first clutch C1 that was operated at the fifth forward speed D5 isreleased and the first brake B1 is operated at the sixth forward speedD6.

As shown in FIG. 3, in a state that the rotation speed of the inputshaft IS is input to the fourth rotational shaft TM4, the firstrotational shaft TM1 is operated as the fixed element by operation ofthe first brake B1. Therefore, the rotation elements of the first andsecond planetary gear sets PG1 and PG2 form the second speed line T2 andchanged rotation speed is output through the third rotational shaft TM3.

In addition, in a state that rotation speed of the third rotationalshaft TM3 is input to the fifth rotational shaft TM5 by operation of thethird clutch C3, the rotation speed of the input shaft IS is input tothe sixth rotational shaft TM6 by operation of the fourth clutch C4.Therefore, the rotation elements of the third and fourth planetary gearsets PG3 and PG4 form a sixth shift line SP6, and D6 is output throughthe seventh rotational shaft TM7 that is the output element.

At this time, the eighth rotational shaft TM8 is operated as the fixedelement, but the third planet carrier PC3 runs idle and has no effectson the shifting since the second clutch C2 is released.

[Seventh Forward Speed]

The first brake B1 that was operated at the sixth forward speed D6 isreleased and the second brake B2 is operated at the seventh forwardspeed D7.

As shown in FIG. 3, in a state that the rotation speed of the inputshaft IS is input to the fourth rotational shaft TM4, the secondrotational shaft TM2 is operated as the fixed element by operation ofthe second brake B2. Therefore, the rotation elements of the first andsecond planetary gear sets PG1 and PG2 form the first speed line T1 andchanged rotation speed is output through the third rotational shaft TM3.

In addition, in a state that rotation speed of the third rotationalshaft TM3 is input to the fifth rotational shaft TM5 by operation of thethird clutch C3, the rotation speed of the input shaft IS is input tothe sixth rotational shaft TM6 by operation of the fourth clutch C4.Therefore, the rotation elements of the third and fourth planetary gearsets PG3 and PG4 form a seventh shift line SP7, and D7 is output throughthe seventh rotational shaft TM7 that is the output element.

At this time, the eighth rotational shaft TM8 is operated as the fixedelement, but the third planet carrier PC3 runs idle and has no effectson the shifting since the second clutch C2 is released.

[Eighth Forward Speed]

The third clutch C3 that was operated at the seventh forward speed D7 isreleased and the first brake B1 is operated at the eighth forward speedD8.

As shown in FIG. 3, since the first and second brakes B1 and B2 areoperated in the first and second planetary gear sets PG1 and PG2, thethird rotational shaft TM3 runs idle even though the rotation speed ofthe input shaft IS is input to the fourth rotational shaft TM4.Therefore, the third rotational shaft TM3 rotates, but the first,second, and fifth rotational shafts TM1, TM2, and TM5 are operated asthe fixed elements.

In addition, in a state that the rotation speed of the input shaft IS isinput to the sixth rotational shaft TM6 by operation of the fourthclutch C4, the fifth rotational shaft TM5 is operated as the fixedelement. Therefore, the rotation elements of the third and fourthplanetary gear sets PG3 and PG4 form an eighth shift line SP8, and D8 isoutput through the seventh rotational shaft TM7 that is the outputelement.

At this time, the eighth rotational shaft TM8 is operated as the fixedelement, but the third planet carrier PC3 runs idle and has no effectson the shifting since the second clutch C2 is released.

[Ninth Forward Speed]

The first brake B1 that was operated at the eighth forward speed D8 isreleased and the first clutch C1 is operated at the ninth forward speedD9.

As shown in FIG. 3, in a state that the rotation speed of the inputshaft IS is input to the first rotational shaft TM1 by operation of thefirst clutch C1, the second rotational shaft TM2 is operated as thefixed element by operation of the second brake B2. Therefore, therotation elements of the first and second planetary gear sets PG1 andPG2 form a fifth speed line T5 and inverse rotation speed is output tothe fifth rotational shaft TM5.

In addition, the rotation speed of the input shaft IS input to thefourth rotational shaft TM4 has no effects on the shifting because thethird rotational shaft TM3 runs idle.

In addition, in a state that the inverse rotation speed is input to thefifth rotational shaft TM5, the rotation speed of the input shaft IS isinput to the sixth rotational shaft TM6 by operation of the fourthclutch C4. Therefore, the rotation elements of the third and fourthplanetary gear sets PG3 and PG4 form a ninth shift line SP9, and D9 isoutput through the seventh rotational shaft TM7 that is the outputelement.

At this time, the eighth rotational shaft TM8 is operated as the fixedelement, but the third planet carrier PC3 runs idle and has no effectson the shifting since the second clutch C2 is released.

[Reverse Speed]

The second brake B2 and the first and second clutches C1 and C2 areoperated at the reverse speed REV.

As shown in FIG. 3, in a state that the rotation speed of the inputshaft IS is input to the first rotational shaft TM1 by operation of thefirst clutch C1, the second rotational shaft TM2 is operated as thefixed element by operation of the second brake B2. Therefore, therotation elements of the first and second planetary gear sets PG1 andPG2 form a fifth speed line T5 and inverse rotation speed is output tothe fifth rotational shaft TM5.

In addition, the rotation speed of the input shaft IS input to thefourth rotational shaft TM4 has no effects on the shifting because thethird rotational shaft TM3 runs idle.

In addition, in a state that the inverse rotation speed is input to thefifth rotational shaft TM5, the eighth rotational shaft TM8 is operatedas the fixed element. Therefore, the rotation elements of the third andfourth planetary gear sets PG3 and PG4 form a reverse shift line RS, andREV is output through the seventh rotational shaft TM7 that is theoutput element.

The planetary gear train according to various embodiments of the presentinvention may achieve nine forward speeds and one reverse speed bycombining four planetary gear sets PG1, PG2, PG3, and PG4 with fourclutches C1, C2, C3, and C4 and two brakes B1 and B2.

The planetary gear train according to various embodiments of the presentinvention may improve power delivery efficiency and fuel economy byachieving multiple-speeds of the automatic transmission.

In addition, since three friction elements operate at each speed and thenumber of non-operating friction elements is minimized, a friction dragloss is decreased. Therefore, drag torque and power loss may be reduced.

The foregoing descriptions of specific exemplary embodiments of thepresent invention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings. The exemplary embodiments were chosen and described in orderto explain certain principles of the invention and their practicalapplication, to thereby enable others skilled in the art to make andutilize various exemplary embodiments of the present invention, as wellas various alternatives and modifications thereof. It is intended thatthe scope of the invention be defined by the Claims appended hereto andtheir equivalents.

What is claimed is:
 1. A planetary gear train of an automatic transmission for a vehicle, comprising: an input shaft into which power of an engine is input; a first planetary gear set including a first sun gear, a first planetary carrier, and a first ring gear; a second planetary gear set including a second sun gear, a second planetary carrier, and a second ring gear; a third planetary gear set including a third sun gear, a third planetary carrier, and a third ring gear; a fourth planetary gear set including a fourth sun gear, a fourth planetary carrier, and a fourth ring gear; a first rotational shaft including the first sun gear, selectively connected to the input shaft, and selectively connected to a transmission housing; a second rotational shaft including the first planetary carrier and the second ring gear, and selectively connected to the transmission housing; a third rotational shaft including the second planetary carrier; a fourth rotational shaft including the second sun gear and directly connected to the input shaft; a fifth rotational shaft including the first ring gear and the fourth sun gear, and selectively connected to the third rotational shaft; a sixth rotational shaft including the third ring gear and the fourth planetary carrier, and selectively connected to the input shaft; a seventh rotational shaft including the third planetary carrier and the fourth ring gear or the fourth ring gear only, and directly connected to an output shaft; and an eighth rotational shaft including the third sun gear and directly connected to the transmission housing.
 2. The planetary gear train of claim 1, wherein each of the first, second, third, and fourth planetary gear sets is a single pinion planetary gear set.
 3. The planetary gear train of claim 1, further comprising: a first clutch interposed between the input shaft and the first rotational shaft; a second clutch interposed between the third planetary carrier and the fourth ring gear; a third clutch interposed between the third rotational shaft and the fifth rotational shaft; a fourth clutch interposed between the input shaft and the sixth rotational shaft; a first brake interposed between the first rotational shaft and the transmission housing; and a second brake interposed between the second rotational shaft and the transmission housing.
 4. The planetary gear train of claim 3, wherein a first forward speed is achieved by operation of the second brake and the second and third clutches, a second forward speed is achieved by operation of the first brake and the second and third clutches, a third forward speed is achieved by operation of the first, second, and third clutches, a fourth forward speed is achieved by operation of the second, third, and fourth clutches, a fifth forward speed is achieved by operation of the first, third, and fourth clutches, a sixth forward speed is achieved by operation of the first brake and the third and fourth clutches, a seventh forward speed is achieved by operation of the second brake and the third and fourth clutches, an eighth forward speed is achieved by operation of the first and second brakes and the fourth clutch, a ninth forward speed is achieved by operation of the second brake and the first and fourth clutches, and a reverse speed is achieved by operation of the second brake and the first and second clutches.
 5. A planetary gear train of an automatic transmission for a vehicle, comprising: an input shaft into which power of an engine is input; a first planetary gear set including a first sun gear, a first planetary carrier, and a first ring gear; a second planetary gear set including a second sun gear, a second planetary carrier, and a second ring gear; a third planetary gear set including a third sun gear, a third planetary carrier, and a third ring gear; and a fourth planetary gear set including a fourth sun gear, a fourth planetary carrier, and a fourth ring gear, wherein the first sun gear is selectively connected to the input shaft and is selectively connected to a transmission housing, the first planetary carrier and the second ring gear are directly connected to each other and are selectively connected to the transmission housing, the second sun gear is directly connected to the input shaft, the first ring gear and the fourth sun gear are directly connected to each other and are selectively connected to the second planetary carrier, the third ring gear and the fourth planetary carrier are directly connected to each other and are selectively connected to the input shaft, the third planetary carrier is selectively connected to the fourth ring gear and an output shaft, and the fourth ring gear and the output shaft are directly connected to each other, the third sun gear is directly connected to the transmission housing.
 6. The planetary gear train of claim 5, further comprising: a first clutch interposed between the input shaft and the first sun gear; a second clutch interposed between the third planet carrier and the fourth ring gear; a third clutch interposed between the second planetary carrier and the first ring gear; a fourth clutch interposed between the input shaft and the fourth planetary carrier; a first brake interposed between the first sun gear and the transmission housing; and a second brake interposed between the first planetary carrier and the transmission housing. 